Test 6: ABS Sensor

The purpose of this test is to evaluate the operation of an Anti-lock Braking System (ABS) Hall Effect Wheel Speed Sensor based on its output voltage and frequency.

ABS Sensor (Hall Effect) Test Setup

Test 6 ABS Sensor Setup
  1. Use the Manufacturer's data to identify the wheel speed sensor's output circuits;
  2. Connect your PicoScope Channel A to the sensor output;
  3. You can also connect other channels to the other ABS sensors if you wish;
  4. Use the Guided Test to set up the PicoScope and to get information about the test:
    • Click on Guided Tests to display the Guided Tests Window;
    • Click the Red Sensors Icon;
    • Choose Wheel Speed;
    • Choose Wheel speed sensor (Hall effect);
    • Click Guide and Settings File;
    • The PicoScope will be setup for the test and a sample waveform will be displayed.
  5. Turn the ignition on but do not start the engine;
  6. Start the PicoScope to see live data;
  7. With the suspect wheel raised, turn it by hand. This will be sufficient to produce output from a good sensor;
  8. With your waveform on the screen, stop the scope;
  9. Use the Waveform Buffer, Zoom and Measurement to examine your waveform;

Inspect the waveform for uniformity across one complete revolution of the road wheel. The wheel speed signal should remain stable and uniform throughout each revolution of the wheel at a fixed speed. PicoScope will reveal anomalies such as poor signal formation, reduced amplitude and sequential changes to each pulse that could reveal ABS pick-up damage.

Test 6 ABS Sensor Wheel Speed Waveform
ABS Sensor showing the reluctor wheel and the sensor

Waveform Notes

In this example, Channel A is connected to the left front wheel and Channel B is connected to the right front wheel.

The hall effect device produces a pulse whenever a reluctor tooth passes the sensor. The voltage level might vary from manufacturer to manufacturer but the frequency of the pulses is proportional to the wheel speed.

ABS to Road Speed Calculator
Tyre Code: / R
Reluctor Teeth
Test Frequency Hz
Tyre Height 96.75 mm
Rim Diameter 431.80 mm
Wheel Height 625.30 mm
Wheel Circumference: 1.96 m
Wheel Speed 10800 revs/h
Road Speed Factor (km/h) 0.14733
Maths Formula (km/h) 0.14733*freq(A)
Road Speed 21.22 km/h
Road Speed Factor (miles/h) 0.09155
Maths Formula (miles/h) 0.09155*freq(A)
Road Speed 13.18 miles/h

Calculating Road Speed (Maths Channel)

One of the most useful but least understood features of PicoScope is the Maths Channel. This feature allows you to enter a formula and the PicoScope will create a new waveform based on that result.

As a demonstration, we have included a tool that creates a formula that calculates road speed in km/h or in miles/hour using an ABS sensor as an input. The vehicle's ECU performs a similar calculation to control the speedometer.

Enter the tyre code and the number of teeth on the ABS sensor sprocket and then enter the formula into a maths channel on the oscilloscope. The PicoScope will display a road speed trace.

The A in the formula is the PicoScope input channel being used. You can change it if you are using a different channel.

For more comprehensive information on how the calculation was done, see our article on Using a Maths Channel to determine Vehicle Speed from ABS.

In the next test, we examine cam and crankshaft waveforms.

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